Biodegradation of the chlorophenoxy herbicide (R)-(+)-mecoprop by Alcaligenes denitrificans

An Alcaligenes denitrificans strain capable of utilizing the herbicide (R)-(+)-2(2-methyl-4-chlorophenoxy)propionic acid (mecoprop) as a sole carbon source was isolated from soil and cultured in liquid medium. Crude cell extracts of the bacterium were utilized in spectrophotometric assays to elucidate a biochemical pathway for degradation of mecoprop. Results indicated a reaction sequence analogous to the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D). GC-MS analysis provided direct evidence for the biotransformation of mecoprop to the transient metabolite 4-chloro-2-methylphenol (MCP). No NADPH-dependent activity was observed during this reaction. Pyruvate was verified as the second product derived from the aliphatic side chain of mecoprop. MCP was subsequently transformed to a substituted catechol by an NADPH-dependent monooxygenase. When grown on mecoprop, A. denitrificans was adapted to oxidize catechol and its 3- and 3-methylated derivatives indicating the broad substrate specificity of catechol dioxygenase. The microorganism was demonstrated to adopt the ortho mechanism of aromatic cleavage which resulted in the formation of 2-methyl-3-carboxymethylene but-2-en-4-olide, a reaction intermediate of the beta-ketoadipate pathway.